Can you use a string to instantiate a class?

Question

I’m using a Builder pattern in Python to separate a bunch of different configuration possibilities. Basically, I have a bunch of classes that are named ID... (e.g. ID12345). These all inherit from the base Builder class. In my script, I need to instantiate an instance for each class (about 50) every time this app runs. So, I’m trying to see if instead of doing something like this:

ProcessDirector = ProcessDirector()
ID12345 = ID12345()
ID01234 = ID01234()

ProcessDirector.construct(ID12345)
ProcessDirector.construct(ID01234)

ID12345.run()
ID01234.run()

Can I do something like this (I know this doesn’t work):

IDS = ["ID12345", "ID01234"]

ProcessDirector = ProcessDirector()
for id in IDS:
  builder = id() #some how instantiate class from string
  ProcessDirector.construct(builder)
  builder.run()

That way, when I need to add a new one in the future, all I have to do is add the id to the IDS list, rather than peppering the new ID throughout the code.

EDIT:

Looks like there are different opinions based on where the data is coming from. These IDs are entered in a file that no one else has access to. I’m not reading the strings from the command line, and I’d like to be able to do as little alteration when adding a new ID in the future.

Asked By: scottm

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Source

Answer 1

There’s some stuff missing from your question, so I’m forced to guess at the omitted stuff. Feel free to edit your question to correct the omissions.

class ProcessDirector( object ):
    # does something

class ID12345( SomeKindOfProcess ):
    pass

class ID001234( SomeKindOfProcess ):
    pass

idList= [ID12345, ID01234]

theProcessDirector = ProcessDirector()
for id in idList:
  builder = id() #Instantiate an object from the class object
  theProcessDirector.construct(builder)
  builder.run()

This works very nicely. It doesn’t instantiate from a string — in practice you don’t often want this. Sometimes, but rarely. More commonly, you a list of class objects from which you want instance objects.

If you actually are getting your class names from the command line, then you’d make the following small change.

validClasses = [ ID12345, ID01234 ]
validMap = dict( ( (c.__name__, c) for c in validClasses ) )
nameStrings = [ "ID12345", "ID01234" ] # from your command-line 
idList= [ validMap[s] for s in nameStrings ]

Everything else remains the same.

[Also, if possible, try to start instance variable names with lowercase letters. Names which start with Uppercase Letters are usually class names.]

Edit

Removed eval. In spite of the fact that eval() is absolutely not a security hole. Eval (and exec and execfile) are only a problem if someone specifically grants access to malicious users.

Answered By: S.Lott

Answer 2

Never use eval() if you can help it. Python has so many better options (dispatch dictionary, getattr(), etc.) that you should never have to use the security hole known as eval().

Answered By: Ignacio Vazquez-Abrams

Answer 3

Simplest way is to just create a dict.

class A(object): 
    pass
class B(object): 
    pass

namedclass = {'ID12345': A, 'ID2': A, 'B': B, 'AnotherB': B,  'ID01234': B}

Then use it (your code example):

IDS = ["ID12345", "ID01234"]

ProcessDirector = ProcessDirector()
for id in IDS:
    builder = namedclass[id]() 
    ProcessDirector.construct(builder)
    builder.run()

Answered By: nosklo

Answer 4

If you wanted to avoid an eval(), you could just do:

id = "1234asdf"
constructor = globals()[id]
instance = constructor()

Provided that the class is defined in (or imported into) your current scope.

Answered By: GoldenBoy

Answer 5

Not sure this is what you want but it seems like a more Pythonic way to instantiate a bunch of classes listed in a string:

class idClasses:
    class ID12345:pass
    class ID01234:pass
# could also be: import idClasses

class ProcessDirector:
    def __init__(self):
        self.allClasses = []

    def construct(self, builderName):
        targetClass = getattr(idClasses, builderName)
        instance = targetClass()
        self.allClasses.append(instance)

IDS = ["ID12345", "ID01234"]

director = ProcessDirector()
for id in IDS:
    director.construct(id)

print director.allClasses
# [<__main__.ID12345 instance at 0x7d850>, <__main__.ID01234 instance at 0x7d918>]

Answered By: dbr

Answer 6

I decided to put together the accepted answer and a comment on the accepted answer. I also added the overloaded __getitem__ so this looks more like a factory pattern.

import sys
import traceback
import ipdb


class CarTypes:
    class Toyota:
        def __repr__(self):
            return "Toyota()"
        def __str__(self):
            return "Instance of Toyota() class"
    class Nissan:
        def __repr__(self):
            return "Nissan()"
        def __str__(self):
            return "Instance of Nissan() class"


class Car:
    def __init__(self):
        self._all_classes = {}

    def construct(self, builder_name):
        setattr(self, builder_name, CarTypes())
        try:
            target_class = getattr(CarTypes, builder_name)
            instance = target_class()
            self._all_classes[builder_name] = instance
        except AttributeError:
            print("Builder {} not defined.".format(builder_name))
            traceback.print_stack()

    def __getitem__(self, type_name):
        return self._all_classes[type_name]

    def car_type(self, type_name):
        return self._all_classes[type_name]


IDS = ["Toyota", "Nissan", "Unknown"]

director = Car()
for id in IDS:
    director.construct(id)

print(director["Toyota"])
print(director["Nissan"])
print(director.car_type("Toyota"))
print(director.car_type("Nissan"))

Edited: I added in some error handling.

Edited: Placed under permissive Creative Commons license. Enjoy.

Answered By: Daisuke Aramaki

Can you use a string to instantiate a class?

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